Speakers and headphones not working

i brought a new acer desktop computer that use vista and everything was fine until today. i turned on my computer and wanted to watch a video on youtube and the sound wouldn't play. i opened my zune software to listen to my music and it wouldn't play. i've checked device manager and my speakers and sound card don't have any yellow exclamation marks next to them. my computer says the device is connected and working and the power source is okay. i don't know what to do anymore so can someone help me before i lose my mind.